There are, basically, two types of olefin polymerization techniques for preparing high molecular weight olefin polymers and copolymers. The oldest commercial technique involves high pressure, high temperature, and the use of a free radical initiator, such as a peroxide; these type polymers are generally known as low density polyethylene (LDPE) and are also known as high pressure polyethylene (HPPE) and as ICI-type polyethylenes. These LDPE polymers contain branched chains of polymerized monomer units pendant from the main polymer "backbone" and generally have densities in the range of about 0.910-0.935 gms/cc.
The other commercially-used technique involves coordination catalysts of the "Ziegler" type or "Phillips" type and includes variations of the Ziegler type, such as the Natta type. These catalysts may be used at very high pressures, but may also (and generally are) used at very low or intermediate pressures. The products made by these coordination catalysts are generally known as "linear" polymers because of the substantial absence of branched chains of polymerized monomer units pendant from the main polymer "backbone", and they are also generally known as high density polyethylene (HDPE). It is modified version of those "linear" polymers to which the present invention pertains. Linear polyethylene (HDPE) ordinarily has a density in the range of 0.941 to 0.965 gms/cc.
In particular, the present invention pertains to "linear" type ethylene polymers wherein ethylene has been polymerized along with minor amounts of alpha, beta-ethylenically unsaturated alkenes having from 3 to 12 carbons per alkene molecule, preferably 4 to 8. The amount of the alkene comonomer is generally sufficient to cause the density of the polymer to be substantially in, or approaching, the same density range as LDPE, due to the alkyl sidechains on the polymer molecule, yet the polymer remains in the "linear" classification; they are conveniently referred to as "linear low density polyethylene" (LLDPE). These polymers retain much of the strength, crystallinity, and extensibility normally found in HDPE homopolymers of ethylene, while also exhibiting characteristics of LDPE such as toughness and low modulus.
The present invention pertains to fine denier fibers and multi-filaments of the LLDPE ethylene copolymers. It is believed that U.S. Pat. No. 4,181,762, U.S. Pat. No. 4,258,097, and U.S. Pat. No. 4,356,220 are representative of the most relevant fiber art of which we are aware. U.S. Pat. No. 4,076,698 discloses methods of producing LLDPE polymers and discloses extrusion of a monofilament.
Convenient references relating to fibers and filaments, including those of man-made thermoplastics, and incorporated herein by reference, are, for example:
(a) Encyclopedia of Polymer Science and Technology, Interscience, New York, Vol. 6 (1967) pp 505-555 and Vol. 9 (1968) pp 403-440;
(b) Man-Made Fiber and Textile Dictionary, published by Celanese Corporation;
(c) Fundamentals of Fibre Formation--The Science of Fibre Spinning and Drawing, by Andrzij Ziabicki, published by John Wiley & Sons, London/New York, 1976;
(d) Man-Made Fibres, by R. W. Moncrieff, published by John Wiley & Sons, London/New York, 1975;
(e) Kirk-Othmer Encyclopedia of Chemical Technology, Vol. 16 for "Olefin Fibers", published by John Wiley & Sons, New York, 1981, 3rd Edition.